diff --git a/CLAUDE.md b/CLAUDE.md
index 3088fbc..cce4b9f 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -92,8 +92,14 @@ will not run under Ray.Di or will break framework expectations.
 7. **Semantic validators**: one class per concept, one `#[Validate]` method,
    throw a domain exception from `src/Exception/`. Link to schema.org in the
    docblock when a standard term exists (`@link https://schema.org/…`).
-8. **Reason services**: always define an `…Interface` and depend on the
-   interface, never the concrete class. Ray.Di binds the implementation.
+8. **Reason services**: Reasons sitting at an **external I/O boundary**
+   (HTTP, DB, payment gateway, third-party API, filesystem, clock, randomness)
+   MUST define an `…Interface`; consumers depend on the interface and Ray.Di
+   binds the implementation — so tests can swap in a Fake. Reasons that are
+   **pure in-process policies or calculators** (rule-book classes with no
+   I/O) MAY be injected as concrete classes; no interface is required.
+   Examples: `PaymentGatewayInterface` and `CreditBureauInterface` (boundary);
+   `IncomePolicy` and `JTASProtocol` (pure policy).
 9. **Namespaces**: follow the existing per-demo pattern
    (`Be\Pattern\<Name>\<Layer>\…` or `Be\App\<Layer>\…`). Never invent a new
    root namespace.